The present invention relates to a system for drilling and logging of a wellbore formed in an earth formation. The invention furthermore relates to a drill string, a logging tool string and a logging collar for use in conjunction with the system of the invention.
U.S. Pat. No. 5589825 discloses a system whereby a logging tool string is lowered through the longitudinal fluid channel of the drill string until the logging tool is located opposite a number of windows provide in the wall of the drill string. Logging signals pass from the logging tool through the windows into the earth formation. Only limited logging information can be obtained with this known system, due to the limited size of the windows. Furthermore, this system requires precise positioning of the logging tool relative to the windows.
U.S. Pat. No. 3112442 discloses a system whereby a logging tool passes through the fluid channel of a drill string until a number of electrodes of the tool pass through the fluid nozzles of the drill bit to below the drill bit. The electrodes are of limited size and have limited capacity for emitting and receiving logging signals. Another drawback of this system is that passage of the electrodes through the nozzles can be hampered by the shape and direction of the nozzles. Furthermore, in case the lower end part of the drill string contains Measurement While Drilling equipment (MWD) or a hydraulic downhole motor, lowering of the logging tool string to the drill bit is virtually impossible.
It is an object of the invention to provide a system for drilling and logging of a wellbore formed in an earth formation, which overcomes the drawbacks of the known systems, and which is robust and provides adequate logging information without hampering drilling operations.
It is another object of the invention to provide a drill string for use in conjunction with the system of the invention.
It is a further object of the invention to provide a logging tool string for use in conjunction with the system of the invention.
It is yet another object of the invention to provide a logging collar for use in conjunction with the system of the invention.
In accordance with the invention there is provided a system for drilling and logging of a wellbore formed in an earth formation, the system comprising a drill string provided with an external recess extending in longitudinal direction of the drill string, a fluid passage for flow of drilling fluid from an upper end of the drill string to a lower end thereof, and a port providing fluid communication between the fluid passage and the recess, the system further comprising a logging tool string capable of passing through the fluid passage and from the fluid passage via the port into the recess, and a removable closure element adapted to selectively close the port.